The use of time-of-flight technology in the field of slow moving vehicles or persons in a narrow and defined zone of interest in close proximity to the sensor is not common. Light travels at slightly less than 1 foot per nanosecond in air. When ranging short distances, such as those required when detecting vehicles in a parking space or adjacent to a traffic signal stop bar, the pulse width needs to be very short, otherwise there will not be a separation between the transmit pulse and receive pulse. For example, aircraft radar's pulse widths tend to be in the microseconds range and they do not work for nearby objects, such as when the planes are low overhead.
If used in a time-of-flight mode, parking sensors need to have a transmit pulse of typically 1-3 nanoseconds duration to have adequate separation between transmit and receive windows. At these pulse widths, the emitted spectrum becomes very broad up to or above 1 GHz.
There is not a greater than 1 GHz block of spectrum anywhere that regulatory agencies like the FCC allocate for such purposes as spectrum is a very valuable resource. At some extremely high frequencies (>60 GHz), there are blocks of spectrum potentially large enough, but they are difficult to use economically with present technologies.
There is a however a “general class” spectrum, under FCC Part 15 (15.209) between intended for such low emissions that there is no possible interference to anyone else. This requires emitters to emit about one trillionth of power of a cell phone. This has been used primarily for emitters that emit inside a closed metal tank (where outside the tank the emissions meet the spec).
The need exists for a sensor using time of flight radar that passes stringent regulatory FCC frequency limits for the first time to detect movement in short distances or close proximities.